The present invention relates generally to programmable implantable pacemakers, and particularly to dual-chamber pacemakers capable of switching from an atrial tracking mode of operation to a non-atrial tracking mode in response to an occurrence of an atrial arrhythmia. More particularly, the present invention relates to a mode switching pacemaker that includes an atrial rate smoothing filter for producing a filtered atrial rate, where the pacemaker automatically switches its mode of operation from an atrial tracking mode to a non-atrial tracking mode in the event the filtered atrial rate exceeds a prescribed upper rate limit.
Essentially, the heart is a pump which pumps blood throughout the body. It consists of four chambers, two atria and two ventricles. In order for the heart to efficiently perform its function as a pump, the atrial muscles and ventricular muscles should contract in a proper sequence and in a timed relationship.
In a given cardiac cycle (corresponding to one "beat" of the heart), the two atria contract, forcing the blood therein into the ventricles. A short time later, the two ventricles contract, forcing the blood therein to the lungs (from the right ventricle) or through the body (from the left ventricle). Meanwhile, blood from the body fills the right atrium and blood from the lungs fills the left atrium, waiting for the next cycle to begin. A typical healthy adult heart may beat at a rate of 60-70 beats per minute (bpm) while at rest, and may increase its rate to 140-180 bpm when the adult is engaging in strenuous physical exercise, or undergoing other physiologic stress.
The healthy heart controls its rhythm from its SA node, located in the upper portion of the right atrium. The SA node generates an electrical impulse at a rate commonly referred to as the "sinus" or "intrinsic" rate. This impulse is delivered to the atrial tissue when the atria are to contract and, after a suitable delay (on the order of 40-80 milliseconds), propagates to the ventricular tissue when the ventricles are to contract.
When the atria contract, a detectable electrical signal referred to as a P-wave is generated. When the ventricles contract, a detectable electrical signal referred to as an R-wave is generated. The R-wave is much larger than the P-wave, principally because the ventricular muscle tissue is much more massive than the atrial muscle tissue. The atrial muscle tissue need only produce a contraction sufficient to move the blood a very short distance--from the respective atrium to its corresponding ventricle. The ventricular muscle tissue, on the other hand, must produce a contraction sufficient to push the blood over a long distance (e.g., through the complete circulatory system of the entire body).
Other electrical signals or waves are also detectable within a cardiac cycle, such as a Q-wave (which immediately precedes an R-wave), an S-wave (which immediately follows an R-wave), and a T-wave (which represents the repolarization of the ventricular muscle tissue).
It is the function of a pacemaker to provide electrical stimulation pulses to the appropriate chamber(s) of the heart (atria or ventricles) in the event the heart is unable to beat on its own (i.e., in the event either the SA node fails to generate its own natural stimulation pulses at an appropriate sinus rate, or in the event such natural stimulation pulses do not effectively propagate to the appropriate cardiac tissue). Most modern pacemakers accomplish this function by operating in a "demand" mode where stimulation pulses from the pacemaker are provided to the heart only when it is not beating on its own, as sensed by monitoring the appropriate chamber of the heart for the occurrence of a P-wave or an R-wave. If a P-wave or an R-wave is not sensed within a prescribed period of time (which period of time is often referred to as the "escape interval"), then a stimulation pulse is generated at the conclusion of this prescribed period of time and delivered to the appropriate heart chamber via a pacemaker lead.
Modern programmable pacemakers are generally of two types: (1) single-chamber pacemakers, and (2) dual-chamber pacemakers. In a single-chamber pacemaker, the pacemaker provides stimulation pulses to, and senses cardiac activity within, a single-chamber of the heart (e.g., either the right ventricle or the right atrium). In a dual-chamber pacemaker, the pacemaker provides stimulation pulses to, and senses cardiac activity within, two chambers of the heart (e.g., both the right atrium and the right ventricle). The left atrium and left ventricle can also be paced, provided that suitable electrical contacts are made therewith.
In general, both single and dual-chamber pacemakers are classified by type according to a three letter code. In this code, the first letter identifies the chamber of the heart that is paced (i.e., the chamber where a stimulation pulse is delivered)--with a "V" indicating the ventricle, an "A" indicating the atrium, and a "D" indicating both the atrium and ventricle. The second letter of the code identifies the chamber where cardiac activity is sensed, using the same letters to identify the atrium or ventricle or both, and where an "0" indicates that no sensing takes place.
The third letter of the code identifies the action or response that is taken by the pacemaker. In general, three types of action or responses are recognized: (1) an Inhibiting ("I") response, where a stimulation pulse is delivered to the designated chamber after a set period of time unless cardiac activity is sensed during that time, in which case the stimulation pulse is inhibited; (2) a Trigger ("T") response, where a stimulation pulse is delivered to the designated chamber of the heart a prescribed period after a sensed event; or (3) a Dual ("D") response, where both the Inhibiting mode and Trigger mode are evoked, inhibiting in one chamber of the heart and triggering in the other.
A fourth letter, "R" is sometimes added to the code to signify that the particular mode identified by the three letter code is rate-responsive, where the pacing rate may be adjusted automatically by the pacemaker based on one or more physiological factors, such as blood oxygen level or the patient's activity level.
Thus, for example, a DVI pacemaker is a pacemaker that paces in both chambers of the heart, but only senses in the ventricle, and that operates by inhibiting stimulation pulses when prior ventricular activity is sensed. Because it paces in two chambers, it is considered a dual-chamber pacemaker. A VVI pacemaker, on the other hand, is a pacemaker that paces only in the ventricle. Because only one chamber is involved, it is classified as a single-chamber pacemaker. Most dual-chamber pacemakers can also be programmed to operate in a single-chamber mode.
Much has been written and described in the art about the various types of pacemakers and the advantages and disadvantages of each. For example, reference is made to commonly assigned U.S. Pat. No. 4,712,555 of Thornander et al., where some helpful background information about pacemakers and the manner in which they interface with a patient's heart is presented. This patent is hereby incorporated by reference in its entirety.
One of the most versatile programmable pacemakers available today is the DDDR pacemaker. This pacemaker represents a fully automatic pacemaker which is capable of sensing and pacing in both the atrium and ventricle, and is also capable of adjusting the pacing rate based on one or more physiological factors, such as the patient's activity level. When functioning properly, the DDDR pacemaker can limit certain drawbacks associated with the use of pacemakers. For example, the DDDR pacemaker can maintain AV synchrony while providing bradycardia support.
In general, DDDR pacing has four functional states: (1) P-wave sensing, ventricular pacing (PV); (2) atrial pacing, ventricular pacing (AV); (3) P-wave sensing, R-wave sensing (PR); and (4) atrial pacing, R-wave sensing (AR). Advantageously, for the patient with complete or partial heart block, the P state of the DDDR pacemaker tracks the atrial rate which is set by the heart's SA node, and then paces in the ventricle at a rate that follows this atrial rate. Because the rate set by the SA node represents the rate at which the heart should beat in order to meet the physiologic demands of the body (at least for a heart having a properly functioning SA node) the rate maintained in the ventricle by such a pacemaker is truly physiologic.
Those skilled in the art have long recognized the advantages of using an atrial tracking pacemaker. For example, U.S. Pat. No. 4,624,260 to Baker, Jr. et al. discloses a microprocessor controlled dual-chamber pacemaker having conditional atrial tracking capability. Similarly, U.S. Pat. No. 4,485,818 of Leckrone et al. discloses a microprocessor-based pacemaker which may be programmed to operate in one of a plurality of possible operating modes, including an atrial rate tracking mode.
Unfortunately, in some instances, a given patient may develop fast atrial rhythms which result from a pathologic arrhythmia such as a pathological tachycardia, fibrillation or flutter. In these cases, a DDDR pacemaker may pace the ventricle in response to the sensed atrial arrhythmia up to the programmed maximum tracking rate (MTR).
Sometimes it is possible at the time of implantation of a pacemaker to determine whether an atrial fibrillation, atrial flutter, or atrial tachycardia condition is going to develop. In such instances, the pacemaker may always be programmed to operate in a different mode of operation, the leads may be repositioned within the heart, or other actions may be taken to minimize the likelihood of such pathologic arrhythmias occurring. Unfortunately, however, it is not always possible at the time of implantation to determine whether a patient will develop an atrial arrhythmia after the pacemaker is implanted.
Therefore, if such pathologic arrhythmias subsequently occur, they must be treated using other techniques, such as through the administration of drugs. Needless to say, the administration of drugs normally requires the attendance of a physician. Unfortunately, however, a physician is not always present when such pathologic arrhythmias develop, and even when a physician is available, such drugs also may undesirably suppress the ability of the SA node to increase the sinus rate during periods of exercise, emotional stress, or other physiologic stress. Thus, the use of such drugs may prevent the pacemaker from functioning as a intrinsic physiologic rate-responsive pacemaker.
As a result, attempts have been made in the art to prevent undesirable tracking of pathologic atrial arrhythmias by automatically switching the pacemaker's mode of operation from an atrial tracking pacing mode to a non-atrial tracking pacing mode. For example, U.S. Pat. No. 4,722,341 of Hedberg et al., teaches an atrium-controlled pacemaker, where the pacemaker temporarily switches from an atrial tracking mode to a non-atrial tracking mode for a fixed number of stimulation pulses if the sensed atrial activity indicates an atrial arrhythmia may be developing. Unfortunately, however, for some patients, a temporary switching from one mode to another without the capability of remaining in the secondary mode for an extended period of time may not be sufficient to correct or arrest the arrhythmia.
In addition, some previously known mode switching techniques are based in whole or in part on the patient's sensed atrial rate exceeding the MTR. This mode switching criterion may cause problems for patients who exhibit normal sinus tachycardia due to physical activity. Another difficulty associated with previously known techniques is that mode switching occasionally occurred due to electrical noise present in the atrial sensing channel of the pacemaker, or due to a one-of-a-kind fast P-wave. In the above instances, rates slightly exceeding the MTR are not indicative of a pathologic arrhythmia. These patients may thus be subjected to undesirably frequent mode switching occurrences as their atrial rates slightly exceed and then drop below the MTR.
This problem was addressed in commonly assigned U.S. Pat. No. 4,944,928 of Sholder, which is hereby incorporated by reference in its entirety. The '928 patent discloses an atrial tracking pacemaker with automatic mode switching capability. The pacemaker's features include the capability of setting a tachycardia rate limit (TRL) slightly above the MTR, so that mode switching to a non-atrial tracking mode occurs when the TRL is exceeded. A third threshold rate is also set at a value below the MTR. The pacemaker switches back to an atrial tracking mode when the patient's atrial rate drops below this third threshold. To avoid mode switching based on a single short atrial interval between atrial events, the atrial rate is continuously averaged over several cycles. This technique effectively prevents frequent mode switches in patients whose atrial rates "hover" around the MTR.
The techniques discussed in the '928 patent represent significant advances over previously known mode switching techniques. However, some concerns remain unaddressed. For example, certain patients may exhibit atrial rates that drastically fluctuate over short periods of time. In such patients, the sensed atrial rate, and even the averaged atrial rate, may frequently exceed the MTR as well as the TRL, and thus result in frequent mode switching even if no pathologic arrhythmia is occurring. In addition, the averaged atrial rate may be distorted if one or more false signals (e.g., electrical noise) are interpreted as atrial events. Finally, for active patients exhibiting a normal sinus tachycardia, a return to the atrial tracking mode set at a threshold rate lower than the MTR may be inappropriate.
Thus, it would be desirable for the pacemaker to switch the pacing mode from an atrial tracking mode to an non-atrial tracking mode only if a pathologic arrhythmia is detected. It would also be desirable to avoid repetitive mode switching based on fluctuations in the sensed atrial rate.
A pacemaker usually has a number of operational parameters which control the pacemaker's performance. Examples of operational parameters include, but are not limited to, the base rate, the AV delay, the atrial and ventricular refractory periods, and the atrial and ventricular sensing configurations. These parameters are typically set initially by the pacemaker manufacturer, but may be changed by a medical practitioner at the time of implantation or during the patient's follow-up visits.
In a typical previously known pacemaker, a mode switch entails changing the primary mode pacing, sensing, and response configurations of the pacemaker to the alternate mode configurations. For example, the primary mode may be DDD, where the pacemaker paces and senses in both the atrial and ventricular chambers, while inhibiting pacing pulses in one chamber and triggering pacing pulses in the other. An alternate mode may be VVI, where the pacemaker paces, senses, and inhibits pacing pulses only in the ventricle. However, since operational parameters are typically defined only for the pacemaker's primary mode of operation, the operational parameters of the pacemaker remain the same after a switch to the alternate mode. This may be undesirable since certain operational parameter settings may cause the pacemaker to perform in a less than optimal manner in the alternate mode, because the settings are usually optimized for performance in the primary mode. Thus, it would be desirable for the pacemaker to have the capability of switching to a different set of operational parameters associated with the alternate pacing mode when the pacemaker switches to the alternate mode, in order to optimize the performance of the pacemaker during the alternate mode.
When initially configuring the pacemaker, the medical practitioner may set the criteria for mode switching, define the primary mode, and select the alternate mode appropriate for the patient. Pacemaker configuration is typically performed soon after the pacemaker is implanted using an implantable device programmer. After implantation, the medical practitioner typically performs periodic follow-up examinations to determine if the pacemaker is operating properly. During these examinations, the medical practitioner can make adjustments to the operational parameters of the pacemaker, and may also adjust the mode switching criteria to improve the performance of the pacemaker. However, it may be difficult to determine if the pacemaker was performing as desired under the previous mode switching criteria. Often the medical practitioner is forced to rely on the patient's description of discomfort to make the determination that the mode switching criteria need to be adjusted. Thus, it would be desirable if the pacemaker could record mode switching events and data pertaining to the mode switching events (e.g., the duration of the switch, the atrial rate at the time of switch, etc.), and store the event and associated data in pacemaker memory for retrieval by the medical practitioner during a follow-up examination.